From what it sounds like, the Cubs goal is to acquire 1 controllable pitcher, with their preference being a SP. They will pivot to a controllable RP if they cannot acquire that. They will then acquire a more mid-tier rental of the other pitching type. So if they grab a controllable SP, they'd go rental RP.
As of right now, the reliever market is churning a bit, but as long as the Cubs are trying for a controlled SP first, they can't really pivot yet to a reliever. If they did, it would likely change how they had to attack SP, which would be backwards for their wants.
3b is a bit different, but the hitter market the Cubs are likely in (Suarez, Castro) isn't really on the move right now.
It's likely that the Cubs will wait this out until they either have to move themselves or a team they're working with accepts they have to budge some.
For teams trying to trade controllable SPs, the reality is that the Cubs are one of the very few games in town who both have need and means, and the Cubs have to know this. While teams with controllable arms don't have to move them, if they wish to cash in now, there aren't many options, either. It's a game of chicken.